DETROIT—Workers began picketing outside five General Motors Corp. plants after the late morning United Auto Workers strike deadline passed Monday, but the union’s national leadership hadn’t publicly announced whether a strike had begun.

The UAW had extended its contract for nine days after it expired on Sept. 14, but the negotiations became bogged down Sunday, apparently over the union’s quest to protect jobs by getting the company to guarantee that new vehicles would be built in U.S. factories.

The UAW hasn’t called a nationwide strike during contract negotiations since 1976, when Ford Motor Co. plants were shut down. There were strikes at two GM plants during contract negotiations in 1996.